Liquid oils used as main starting materials for salad oils and the like, for example, soybean oil, rapeseed oil, corn oil, rice oil, cottonseed oil, safflower oil, kapok oil, peanut oil, sunflower oil and the like, are cheaper than so-called vegetable butter such as cacao butter, shea butter, sal fat and the like. However, in general, they are more expensive than palm oil and its fractionated oils. Accordingly, there have been many proposals aimed at reduction of cost of starting fats and oils of salad oils by using palm oil or fractionated oils thereof as a part of the starting materials.
These proposals include Japanese Patent Laid Open Publication Nos. 107304/1974, 63403/1978 (WPI Acc No. 78-50744A/28), 110246/1985 (WPI Acc No. 85-181373//30), 293389/1986 (WPI Acc No. 87-039021/06), 296096/1986 (WPI Acc No. 87-040195/06) and the like. Among them, the specification of Japanese Patent Laid Open Publication No. 198992/1988 (WPI Acc No. 88-273897/39) describes that there are various drawbacks in most of these prior arts.
Further the description of Japanese Patent Laid Open Publication No. 198992/1988 itself proposes a process for production of a liquid oil comprising subjecting a mixture of a solid fat such as palm oil, palm stearin, palm olein or the like and a liquid oil such as rapeseed oil, palm fractionated liquid oil or the like to enzymatic ester interchange with a lipase having 1,3-selectivity as a catalyst. As the "keystone" of the proposal, this patent application asserts that selective ester interchange results in decrease in triglycerides containing two or more saturated fatty acids, that is, decrease in triglycerides of SSS and SSU (S and U used herein mean a saturated fatty acid residue and an unsaturated fatty acid residue constituting glycerides, respectively; this expression does not limit the position of bonding of the residues) and, thereby, low-temperature resistance must be improved. Further, the application discloses that, after ester interchange, the conventionally required fractionation operation is almost unnecessary.
Thus, palm oil, even a low-melting point fraction known as palm olein, contains a solid component and, in order to use the oil as a part of the starting materials of liquid oil products, it is necessary to improve its low-temperature resistance. In this respect, according to the present inventors' knowledge, the problem of "turbidity" produced after storage at a low temperature is not yet solved by the method of Japanese Patent Laid Open Publication No. 198992/1988.
The present inventors have also studied fats and oils obtained by mixing palm olein with a liquid oil and subjecting the mixture to 1,3-selective ester interchange, and recognized that the process, wherein palm oil and a liquid oil are compounded and 1,3-selective ester interchange is carried out, is not suitable for the production of a salad oil. This recognition is based on the following present inventors' finding:
(1) The ester interchanged oil obtained is inevitably accompanied with problems of causing turbidity upon storage at a low temperature for a long period of time and, in addition, causing oil separation due to demulsification when mayonnaise is produced by using the resulting salad oil and stored in a refrigerator;
(2) When the microscopic observation of the ester interchanged oil itself was conducted after standing at 5.degree. C. for about one month to investigate the cause of demulsification, coarse needle crystals grown like bursa of chestnut were found. These crystals are supposed to cause destruction of the interface; and
(3) Even in the case of the ester interchanged oil obtained by using palm olein, the existing SSU level is not low. On the contrary, SSS content is rather increased after ester interchange.
The present inventors have an idea that, although removal of a crystallized component which causes turbidity is inevitable unless post-fractionation is conducted, modification of crystals of an ester interchanged oil can be employed in order to use the oil in the production of an emulsified food such as mayonnaise to prevent demulsification due to storage at a low temperature, rather than removal of the crystallized component (i.e., the use as a salad oil) which entails higher expenditure. The present inventors have done various studies based on this idea. As a result, it has been found that, by subjecting palm olein and a laurin fat or oil to ester interchange, growth of coarse crystals can be efficiently inhibited and the ester interchanged oil obtained can be used for emulsified food which is exposed to a low temperature while preventing demulsification, although the amount of crystals produced is not necessarily decreased and, on the contrary, the melting point and cloud point thereof are raised.